Process and apparatus for electroplating substrates

ABSTRACT

An apparatus for electroplating one or more surfaces ( 2,3 ) on one or more substrates ( 1 ), especially solar cells ( 1   a ), is described. The apparatus includes an electrochemical coating bath ( 13 ), which has a coating tank ( 12 ) filled with an electrochemical coating liquid ( 14 ). The apparatus also includes a conveying device ( 15 ) for transporting the substrate through the coating bath ( 13 ), a light source circuit ( 60 ) with light sources ( 64 ) for irradiating the substrate ( 1 ) and an electrolytic cell rectifier circuit ( 50 ) for the substrate with anodes ( 54 ). The apparatus is characterized by a device for generating synchronous current pulses and light pulses, so that during a time interval between the current pulses the irradiating of the substrate or substrates is interrupted. A process for electrochemical plating of the surface of the substrate or substrates is also described.

CROSS-REFERENCE

The subject matter described and claimed herein below is also describedin German Patent Application No. 10 2009 029 551.8, filed on Sep. 17,2009 in Germany. This German Patent Application provides the basis for aclaim of priority of invention for the invention described and claimedherein below under 35 U.S.C. 119 (a)-(d).

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to a process for electroplating at leastone surface of at least one substrate, especially solar cells, in whichthe substrate is moved through an electrolyte coating liquid, isirradiated with light and an electroplating current is applied to thesubstrate by means of an electrolytic cell rectifier circuit.

It also relates to an apparatus for performing the process, comprisingan electrochemical coating bath, which comprises a coating tank filledwith an electrochemical coating liquid, a conveying device fortransporting the substrate through the coating bath, a light sourcecircuit with at least one light source for irradiating the substrate andan electrolytic cell rectifier circuit for the substrate with an anode.

2. The Description of the Related Art

DE 10 2007 038 120 A1 describes a type of coating apparatus, in whichsolar cells are conveyed through a coating tank, which contains acoating bath, by means of conveying rollers. Different types of lightsources, e.g. LEDs, whose wavelengths are adjusted or set according tothe respective coating liquid, are arranged on the underside of thetank.

The coating can be exclusively light induced, but also can be currentassisted. The irradiation with the light sources generates a voltage inthe cell, which induces a current flow. This current flow initiates adeposition of metal from the coating bath on the front side of the solarcell. The solar cell is the cathode in the electroplating currentcircuit, which is characterized as an electrolytic cell rectifiercircuit. However the combined light- and current-generated coatingprocess produces no significant increase of throughput.

DE 42 25 961 A1 teaches that operating with constant direct voltage inan electroplating circuit is harmful. The object to be coated is locatedin substantially the same electric field on its path through the coatingbath. The plating speed, i.e. the speed with which the deposited metallayer is built up on the substrate, is however comparatively small. Thatmeans that the length of the electrochemical plating apparatus for agiven speed of movement of the object must be very great.

In order to achieve a greater plating speed the electrochemical circuit,i.e. the electrolytic cell rectifier circuit, is operated with a pulsedcurrent, as proposed in DE 42 25 961 A1. It has been said that theplating speed can then be increased many times with these measures. Thecurrent-free time intervals between the current pulses are compensatedfor by increasing the current applied during the current pulse.

SUMMARY OF THE INVENTION

It is an object of the present invention to increase the throughput ofthe substrate through the coating apparatus as well as the speed of thecoating process.

According to a first aspect of the invention the apparatus forelectroplating a substrate according to the invention comprises anelectrochemical coating bath, which has a coating tank filled with theelectrochemical coating liquid; a conveying device for transporting thesubstrate through the coating bath; a light source circuit with at leastone light source for irradiating the substrate; an electrolytic cellrectifier circuit for the substrate with an anode and means forproducing synchronous current pulses and light pulses, so that during atime interval between the current pulses the irradiating is interrupted.

According to a second aspect of the invention the process forelectroplating the substrate according to the invention comprises:

-   -   a) moving the substrate through an electrolytic coating liquid;        -   b) during the moving of the substrate irradiating the            substrate with light;        -   c) during the irradiating passing an electroplating cu rent            through the substrate by means of an electrolytic cell            rectifier circuit; and        -   d) pulsing the electrical current and the light            synchronously, so that during a time interval between            current pulses the irradiating is interrupted.

The apparatus for the invention is particularly characterized by meansfor producing synchronous current pulses and light pulses, so that theirradiation is interrupted between the current pulses.

It has been shown that the throughput can be increased when the processnot only uses current pulses but also light pulses synchronized with thecurrent pulses. Synchronous pulses means that the light pulse is alwaysapplied to the substrate when the current pulses is applied. The pulseinterval between the current pulses always coincides with the pulseinterval between the light pulses. In order to interrupt the irradiationof the substrate surface to be coated, for example the light source canbe shut off. Also it is possible to mask the light source during thepause interval. LEDs are the preferred light sources.

The plating speed could be still further increased many times with thesynchronous light pulses in comparison to the constant irradiationaccording to DE 10 2007 038 120 A1.

Preferably the means for synchronous production of current pulses andlight pulses comprises at least one pulse generator in the electrolyticcell rectifier circuit and/or in the light source circuit.

The electrolytic cell rectifier circuit and/or the light source circuitare preferably connected to the pulse generator.

A pulse generator can be provided to which both circuits are connected.Also an individual pulse generator can be provided in each circuit. Theindividual pulse generators in each circuit would be connected togetherfor production of synchronous pulses.

Preferably the light source circuit is coupled by means of anoptocoupler with the electrolytic cell rectifier circuit. Because of thegalvanic separation of the input and the output the coupling provided bythe optocoupler has the advantage that interfering effects from onecircuit will not be transmitted to the other circuit. The coupling ofthe circuits by means of an optocoupler thus provides the advantage of agreater stability and uniformity of the generated pulses.

According to another embodiment of the apparatus according to theinvention the light source circuit and/or the electrolytic cellrectifier circuit have at least one control device, which can controlthe light intensity of the light source and/or the strength of orcurrent applied during the current pulses according to the widths of thecurrent pulses and/or the light pulses. The morphology of the depositedmetal layers can be influenced by means of the at least one controldevice via the respective heights of the light pulses and the currentpulses. Graduated metal layers with optimized properties can bedeposited. Another possibility consists of simultaneous variation of theheights of the light pulses and the current pulses, whereby the heightof the current pulses is decreased and the height of the light pulses issimultaneously increased during the electrochemical plating process.

In order to further increase the plating speed and thus the throughput,means for producing a coating liquid flow are provided. Since the atleast one substrate is moved through the resting coating liquid, a flowis already present at the at least one surface of the at least onesubstrate. When the means for producing a coating liquid flow isoperated, an additional flow over the at least one surface of the atleast one subtrate in addition to the flow due to the substrate motionis thus provided.

Indeed when the current density is increased, e.g. by increasing theheight of the current pulses, the throughput is indeed increased,because the coating thickness to be attained is reached sooner. Howeverthe current density cannot be arbitrarily increased, because theso-called limiting current density must be considered. The term“limiting current density” means the current density at which the freemetal ion concentration at the cathode surface to be coated, approacheszero.

When the limiting current density is exceeded, gaeous hydrogen isproduced because of depletion of the metal ions in the electrolyte,which produces, among other things, pores in the prevoiusly depositedmetal layer, with the consequence that the morphology of the metal layeris impaired, which can lead to pulverization of the metal layer.

It has been shown that the higher the flow speed of the coating liquidat the surface of the cathode area of the at least one substrate, thehigher the limiting current density.

According to the first embodiment the means for producing the flow ofthe coating liquid comprises at least one nozzle for conducting thecoating liquid. Because of the use of the at least one nozzle thecoating liquid flow is advantageously a turbulent flow, which ispreferably so large that it prevails or reinforces the flow caused bythe transport of the substrate through the coating liquid at the surfaceto be coated.

Preferably the at least one nozzle is arranged in the coating bathopposite to the surface to be coated.

The at least one nozzle is preferably oriented perpendicular to the atleast one surface of the at least one substrate to be coated. Because ofthat a flow of coating liquid is directed to the at least one surface ofthe at least one substrate so that whirls or eddies in the flow areproduced at the substrate. These eddies or whirls at the surface of thesubstrate should have a speed which is greater than the relative speedof the coating liquid caused by the transport of the substrate.

Preferably the nozzles are arranged between the anodes of theelectrolytic cell rectifier circuit. The liquid fed from the nozzles canthen flow unhindered to the surface to be coated and the flow is notimpaired by the presence of the anodes.

The nozzles are preferably Venturi nozzles, with which a high outletspeed of the coating liquid can be attained.

According to a second embodiment the means for producing the flow speedof the coating liquid is a circulating device.

This circulating device is preferably a countercurrent flow device. Thecountercurrent device produces a flow which is in a direction that isopposite to the transport direction of the substrate in the coatingbath. The inlet of the coating tank is preferably arranged at the outletof the transport device and the outlet of the coating tank is preferablyarranged at the inlet of the transport device. The coating liquid ispreferably pumped in a direction that is opposite to the transportdirection of the substrate. Preferably the inlet and outlet of thecounter-current flow device are at the same height as the transportdevice, i.e. arranged at the height of the substrate to be transported,so that the counter flow is not hindered by the other structures in thecoating bath.

The device with the nozzles can similarly be used alone as thecirculating device. It is however preferred to provide both devices incombination with each other.

The process for electroplating at least one surface of the at least onesubstrate is characterized in that the electroplating current and thelight are synchronously pulsed, wherein the irradiation with the lightis interrupted in the time interval between the current pulses.

The respective widths of the light pulses and the current pulsesadvantageously amount to from 0.1 ms to 10000 ms. Preferred pulse widthsare 1 ms to 1000 ms, particularly 1 ms to 100 ms.

The pulse widths are preferably selected to be equal to the timeinterval between the pulses.

The current pulses and/or the light pulses are preferaly rectangularpulses.

The heights and the widths of the light pulses and/or the current pulsescan be varied. The variation of the pulses permits deposition of e.g.graded metal layers with optimized properties. In this way individualplating programs may be realized by means of the control device orcontrol devices.

It has been shown that it is preferable to reduce the pulse heights ofthe light pulses when the pulse widths are increased. Thus for exampleif the light intensity is 10% of the maximum light intensity when thepulse width is 100 ms, the light intensity can be 20% of the maximumwith a pulse width 1 ms and 80% with a pulse width of 0.5 ms.

To improve the throughput a direct current can be applied in theinterval between the current pulses, whose strength I₁ is less than orequal to 0.5×I₂ wherein I₂ is the pulse height of or current appliedduring the current pulses. However the value I₁ of the current duringthe time interval between the current pulses is preferably low enough sothat the required regeneration of the electrolytic coating liquid and/orthe metal concentration at the cathode surface to be coated is notimpaired.

The coating liquid is preferably put into a counter flow opposite to thefeed direction of the at least one substrate in the vicinity of the atleast one surface of the at least one substrate to be coated.

In addition to this counter flow or even independently of this counterflow a turbulent flow of this coating liquid can be produced at least inthe vicinity of the at least one surface of the at least one substrateto be coated.

BRIEF DESCRIPTION OF THE DRAWING

The objects, features and advantages of the invention will now beillustrated in more detail with the aid of the following description ofthe preferred embodiments, with reference to the accompanying figures inwhich:

FIG. 1 is a vertical cross-sectional view through a coating tankaccording to the invention;

FIG. 2 is a circuit diagram for and a diagrammatic view of oneembodiment of an arrangement of the light sources;

FIG. 3 is a circuit diagram for and a diagrammatic view of anotherembodiment of an arrangement of the light sources; and

FIGS. 4 and 5 are respective graphical illustrations of the correlationbetween current and light intensity pulses in the process according tothe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An apparatus 10 for coating substrates 1 is illustrated in FIG. 1. Solarcells 1 a are used as substrates in the present example. The apparatus10 includes a coating tank 12, which comprises a coating bath 13, whichcontains an electrolytic coating liquid 14.

A conveying device 15, which comprises upper conveying rollers 16 andlower conveying rollers 18, is provided in the upper part of the coatingtank 12. The solar cells 1 a are held between the upper and lowerconveying rollers 16, 18 and conveyed in the direction of the arrow 5.Since the coating tank 12 is filled with the coating liquid 14, thesolar cells 1 a are completely within the coating bath 13.

A circulation device 30 for producing a flow in the coating bath 13,which has a first liquid conductor 36, is provided. The coating tank 12has a first outlet 22 below the conveying device 15, to which the firstliquid conductor 36 is connected. The coating liquid 14 is pumped fromthe coating tank 12 via the first outlet 22 and fed back into thecoating tank 12 through a first inlet 20, which is arranged in thebottom region of the coating tank 12, by means of a first pump 32arranged in this first liquid conductor 36.

An inlet flow system 40 with an inlet pipe 42 in the interior of thecoating tank 12 is connected at one end to the first liquid conductor36. At the other end of the inlet pipe 42 a horizontally orienteddistributor pipe 44 is connected, in which a plurality of Venturinozzles 46 are provided. These Venturi nozzles 46 are orientedvertically and thus arranged perpendicular to the solar cells 1 a.

The input coating liquid flows with a high speed from these Venturinozzles 46 and thus impinges substantially perpendicularly on the facingor front side 3 of the solar cells 1 a to be coated from below (see FIG.2).

So that this flow of coating liquid is not disturbed by parts orcomponents of the apparatus, these Venturi nozzles 46 are arrangedbetween the anodes 54 of the direct current circuit 50 (see FIG. 2).Light sources 64, which are LED light strips, are arranged above theanodes 54. The arrangement of light strips is only illustratedschematically. Especially this leads to turbulent flow on the surface 3of the solar cells 1 a, so that the coating liquid 14 can regeneraterapidly in the vicinity of the surface 3 to be coated during theinterval between the pulses.

In order to produce a flow opposite to the conveying direction 5 withinthe coating bath the coating tank 12 has a second outlet 23 in its leftlower region. A second liquid conductor 38 is connected to this secondoutlet 23. A second pump 34 is provided in the second liquid conductor38. This conductor 38 opens into a second inlet 21 in the upper rightregion of the coating tank 12. The second inlet 21 is located in thevicinity of the solar cells 1 a, so that a horizontal flow in adirection opposite to the conveying direction 5 is produced between thesecond inlet 21 and the first outlet 22.

The apparatus 10 for example can be equipped without the second liquidconductor 38. Another embodiment of the apparatus can be designedwithout the inlet flow system 40 with and without the second liquidconductor 38. In the embodiment without the second liquid conductor 38the first liquid conductor 36 is preferably connected to the secondinlet 21 to the coating tank 12.

A solar cell 1 a is illustrated in the detailed cross-sectional viewthat is part of FIG. 2. It has a metallization on its back side 2 andstrips 4 comprising a suitable paste for formation of contact fingers onits front side 3. Screen printing paste is advantageously used forformation of the electrode structures. Metal is deposited from theelectrochemical coating liquid only in the vicinity of this screenprinting paste during the electrochemical deposition.

The upper conveying rollers 16 contact the metallized rear side 2 of thesolar cells 1 a and can be used for application of an electrochemicalcurrent. For this purpose an electrolytic cell rectifier circuit 50 isprovided, which connects the upper transport rollers 16 with the anodes54, which are preferably silver anodes. A first voltage source 52 and apulse generator 53 are provided in the electrolytic cell rectifiercircuit 50. Current pulses are applied to the solar cells 1 a and theanodes 54 by means of the pulse generator 53.

Furthermore a light source 64, e.g. a LED, is shown in FIGS. 2 and 3 andrepresents a plurality of light sources for irradiation of the surface 3of the substrate to be coated. This light source 64 is connected in alight source circuit 60, which has a second voltage source 62.

Both circuits 50 and 60 are coupled with each other by means of anoptocoupler 56. The input 57 of the optocoupler 56 is connected to theelectrolytic cell rectifier circuit 50 and the outlet 58 of theoptocoupler is connected to the light source circuit 60.

The optocoupler 56 is switched in such way that the light source 64 isturned on at the same time that a current pulse is generated, so thatthe light pulse is produced simultaneously with the current pulse.During the time interval between the current pulses the light source 64is turned off.

In FIG. 3 an additional embodiment of the apparatus according to theinvention is illustrated, in which no optocoupler 56 is provided.Instead of the optocoupler 56 the light source circuit 50 is connecteddirectly to the pulse generator 53, which thus produces the pulse forboth circuits 50 and 60.

Optionally a second control device 66 can be provided in the lightsource circuit 60. The control device 66 can control the light pulsestrength for example according to the length of the pulse, i.e. thepulse width.

Similarly a control device 59 can be provided, with which current pulseshaving different pulse widths and height can be generated. A completelyautomatic electroplating program with individual coating steps may beprovided by means of both control devices 59, 60. This is of specialadvantage in order to form graduated layers on which specific electrodestructures are formed by the paste 4.

Two diagrams of the current pulses and the light pulses are illustratedin FIG. 4. The light pulses and the current pulses are of equal length.Respective time intervals are provided between the current pulses andthe light pulses, which are of the same length as the pulse widths. Thelight pulses and the current pulses are completely synchronized. Thelight intensity is zero between the pulses in the time interval. Inorder to achieve this the light source can be turned off or maskedduring the time interval.

Another embodiment of the process according to the invention isillustrated in FIG. 5, in which a direct current, with a value I₁, isapplied during the time interval between the current pulses. Thecurrent, I₁, amounts to 50% of the current I₂, which is applied to thesubstrate during the current pulses. The current pulse in the embodimentof FIG. 5 is the same height as that shown in FIG. 4.

PARTS LIST

-   -   1 substrate    -   1 a solar cell    -   2 rear side    -   3 front side    -   4 paste    -   5 conveying direction    -   10 apparatus    -   12 coating tank    -   14 electrolytic coating liquid    -   15 conveying device    -   16 upper conveying rollers    -   18 lower conveying rollers    -   20 first inlet    -   21 second inlet    -   22 first outlet    -   23 second outlet    -   30 circulating device    -   32 first pump    -   34 second pump    -   36 first liquid conductor    -   38 second liquid conductor    -   40 inlet flow system    -   42 inlet pipe    -   44 distributor pipe    -   46 Venturi nozzle    -   50 electrolytic cell rectifier circuit    -   52 first voltage source    -   53 pulse generator    -   54 anode    -   56 optocoupler    -   57 input of the optocoupler    -   58 output of the optocoupler    -   59 first control device    -   60 light source circuit    -   62 second voltage source    -   64 light source, LED    -   66 second control device

While the invention has been illustrated and described as embodied in aprocess and apparatus for electroplating substrates, it is not intendedto be limited to the details shown, since various modifications andchanges may be made without departing in any way from the spirit of thepresent invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed is new and is set forth in the following appendedclaims.

1. An apparatus for electroplating at least one surface (2,3) of atleast one substrate (1), said apparatus comprising an electrochemicalcoating bath (13), which has a coating tank (12) filled with anelectrochemical coating liquid (14); a conveying device (15) fortransporting the at least one substrate through the coating bath; alight source circuit (60) with at least one light source (64) forirradiating the at least one substrate (1); an electrolytic cellrectifier circuit (50) for the at least one substrate with anodes (54);and means for producing synchronous current pulses and light pulses, sothat during a time interval between said current pulses the irradiatingwith the at least one light source is interrupted.
 2. The apparatus asdefined in claim 1, wherein the means for producing synchronous currentpulses and light pulses comprises at least one pulse generator (53) inthe electrolytic cell rectifier circuit (50) and/or in the light sourcecircuit (60).
 3. The apparatus as defined in claim 2, wherein saidelectrolytic cell rectifier circuit (50) and said light source circuit(60) are electrically connected to said at least one pulse generator(53).
 4. The apparatus as defined in claim 1, wherein the light sourcecircuit (60) is coupled with the electrolytic cell rectifier circuit(50) by an optical coupler (56).
 5. The apparatus as defined in claim 1,wherein the light source circuit (60) and/or the electrolytic cellrectifier circuit (50) has at least one control device (59, 66), whichcontrols a light intensity of the at least one light source (64) and/oramplitudes of the current pulses in accordance with respective widths ofthe current pulses.
 6. The apparatus as defined in claim 1, furthercomprising means for producing a flow of the electrochemical coatingliquid (14).
 7. The apparatus as defined in claim 6, wherein said meansfor producing the flow of the electrochemical coating liquid (14)comprises at least one nozzle for conducting the electrochemical coatingliquid (14).
 8. The apparatus as defined in claim 7, wherein the atleast one nozzle is arranged in the coating bath (13) opposite to the atleast one surface (2,3) of the at least one substrate (1) to be coated.9. The apparatus as defined in claim 7, wherein the at least one nozzlein the coating bath (13) is oriented in a direction perpendicular to theat least one surface (2,3) of the at least one substrate (1) to becoated.
 10. The apparatus as defined in claim 7, wherein the at leastone nozzle is arranged between the anodes (54) of the electrolytic cellrectifier circuit (50).
 11. The apparatus as defined in claim 7, whereinthe at least one nozzle is at least one Venturi nozzle (46).
 12. Theapparatus as defined in claim 6, wherein said means for producing theflow of the electrochemical bath liquid (14) comprises a circulatingdevice (30).
 13. The apparatus as defined in claim 12, wherein saidcirculating device (30) is a countercurrent flow device.
 14. A processfor electroplating at least one surface of at least one substrate, saidprocess comprising the steps of: a) moving the at least one substratethrough an electrolytic coating liquid; b) during the moving of the atleast one substrate irradiating the at least one substrate with light;c) during the irradiating applying an electric current to the at leastone substrate by means of an electrolytic cell rectifier circuit; and d)pulsing the electrical current and the light synchronously, so thatduring a time interval between current pulses the irradiating isinterrupted.
 15. The process as defined in claim 14, wherein pulsewidths of light pulses produced by the pulsing of the light and of thecurrent pulse are each from 0.1 ms to 10,000 ms.
 16. The process asdefined in claim 14, wherein said time interval between the currentpulses is equal to a pulse width of said current pulses.
 17. The processas defined in claim 15, wherein the current pulses and the light pulsesare rectangular-shaped pulses.
 18. The process as defined in claim 15,wherein the light pulses and the current pulses have respective pulsewidths and/or respective amplitudes and said respective pulse widthsand/or said respective amplitudes are varied during the process.
 19. Theprocess as defined in claim 15, wherein the more the amplitudes of thelight pulses are reduced, the greater are the pulse widths of the lightpulses.
 20. The process as defined in claim 14, further comprisingapplying a direct current (I₁) to the at least one substrate during saidtime interval between the current pulses that is less than or equal toone half of a current (I₂) applied during the current pulses.
 21. Theprocess as defined in claim 14, wherein the electrochemical bath liquidflows in a direction that is opposite to a conveying direction (5) ofthe at least one substrate at least in the vicinity of the at least onesurface of the at least one substrate to be coated.
 22. The process asdefined in claim 21, further comprising a turbulent flow of theelectrochemical bath liquid at least in the vicinity of the at least onesurface of the at least one substrate to be coated.